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Precursor of teeth and bones discovered in 810-million-year old fossils

Single-celled fossils found in Canada show the earliest evidence of a tissue-hardening process known as biomineralisation, writes Andrew Masterson.

Progressively more zoomed scanning electron micrographs of the single-celled microfossils, showing at right the hydroxyapatite filaments.

Cohen et al.

Hardened types of tissue are found in a huge number of animal phyla – think bones and teeth in mammals, shells in molluscs – but when the ability to create such useful structures first evolved has long been something of a mystery.

Until now. Research published in the journal Science Advances reveals convincing evidence that tissue-hardening, known as “biomineralisation”, was occurring in a single-celled organism as early as 810 million years ago.

Scientists led by Phoebe Cohen of Williams College in Williamstown, Massachusetts, US, found evidence of mineralisation in minute fossils lodged in a rock formation called the Fifteenmile Group, in the Ogilvie Mountains in the Yukon region of Canada.

The fossils are of eukaryotic cells – regarded as the first complex organisms. Every living thing except bacteria and archaea is made of eukaryotes – cells that feature discrete organelles – including a nucleus – contained within membranes.

To make their discovery, Cohen and colleagues studied unicellular fossils found in a large slab of mud, slate and limestone.

Looking at the samples through a high-resolution microscope the team found signs of long, thin mineralised fibres – shapes that indicated a biological rather than geological origin.

Dating placed the fossils at around 810 million years old, placing them pretty much in the middle of the Neoproterozoic Era, which ran from about 1000 million years ago to the dawn of the Paleozoic Era about 550 million years later.

At that time, Fifteenmile area would have been under the ocean. Cohen and her colleagues suggest that the water would have been high in dissolved calcium phosphate – a necessary compound for the advent of hardened tissue.

The chemical load, the researchers write, may have provided the “opportunity for eukaryotes to explore biomineralisation for what may have been the first time in Earth’s history.”